Noise suppression circuits



Feb- 27, 1945 J. A. woRciEs-rERQ-JR 370,216

l -NoIsE SUPPRESSION CIRCUITS Filed July 22, 1942 SECOND L mTER l 7l 5B 0 76 7; rgy 59 :2 e:

Inventor:

Josph Awocegter y His Attorney.A

Patented Feb. 27, 1945 2,370,216 N OISE SUPPRESSION CIRCUITS Joseph A. Worcester, Jr., Faireld, Conn., assignor to General of New York Electric Company, a corporation Application Juiy zz, 1942. seran No. 451,847

(ci. 25o-2o) l 18 Claims.

My invention relates to noise suppression circuits, and it is particularly applicable to noise suppression circuits for automatically rendering the output circuits of a frequency modulation receiver inoperative when no signals are being received or when the level of a received signal is below a predetermined selectable value. More specifically, the present invention is an improvement upon the arrangement shown in my coopending application Serial No. 407,543, filed August 20, 1941, and assigned to the same assignee as the instant application. V

lIt is well known that objectionable noises may be emitted by the signal reproducer of a radio receiver during those intervals when no signals are being received or while the receiver is being tuned between carrier channels. These noises may arise from a great many causes including cross-modulation effects, natural at-I mospheric static, locally produced high frequency electrical disturbances; shot" effects, and ther# mal agitation of the tubes and circuits of the receiver itself, ete. Therefore, various squelch circuits have heretofore been. proposed for muting the sound reproducer except when the receiver is tuned to a signal of sufficient strength to override the noise level and provide satisfactory reproduction. The operation of such circuits has variously been designated by such terms as background noise suppression, interchan nel noise suppression, and. carrier-01T noise suppression. v

It is well known that a properly designed irequency modulation receiver inherently provides a high signal to noise, ratio so long as the received signals exceed a predetermined level. This is due in large measure to the action of the amplitude limiting circuits which transmit the desired frequency modulation but 'discriminate against amplitude modulation. Thus so. long as any signal suiliciently strong to operate the limiter is received, any noise voltages which cause undesired amplitude modulation of the signal are reduced to a very low level at the limiter output and do not cause objectionable disturbances in the sound reproducenAccordingly, noise suppression or squelch circuits heretofore proposed have 4been based upon the principle that the sound reproducer should be muted until the signal voltages appear which are of themselves suciently strong Asubstan tially to saturate the limiter.

In certain applications, however, it has been found desirable to provide a certain degreeof adjustability so thatl the cuto point of the sound reproduce'r disabling means with respect to the strength of a received signal may be varied at will, thereby to permit the reception of weak signal voltages in spite of the fact that a certain amount of amplitude modulated noise voltage is also reproduced. Itis also desirable to provide means for substantially instantaneously disabling the noise suppressionv circuit as vsoon as a signal voltage of the selected strength ap- Pears.

Accordingly, it isa general object of my inventicn to provide anew and improved carriervo1 noise suppression circuit which is particularly adapted to the requirements of a frequency modulation receiver.

Itis a further object of my invention to provide a new and improved carrier-oil noise suppression circuit which is operative automatically to'squelch or mute the sound reproducer so long as the amplitude of the `frequency modulated carrier wave is below any predetermined selectable value.

It is a still further object of my invention to suppression circuit having means for rendering f the circuit completely and substantially instantaneously inoperative upon theoccurrence of a signal voltage of predetermined value..

It is another object of my invention to provide an improved carrier-oil noise suppression circuit whose cuto'il* point with respect to the amplitude of a received frequency modulated' carrier voltage is selectable and which is very sensitive in that its noise suppression eifect disy appears substantially instantaneously upon the attainment by the carrier of the preselected level. i

In a preferred embodiment of my invention the noise voltages supplied to the limiter under no-signal orweak signal conditions are detected andD utilized to provide bias potentials for reducing the transfer elclency of a portion of the transmission channel following the Preferably one of the amplifying stages following the limiter is completely blocked and the sound Yreproducer thereby eiectively muted. If the noise voltage indication is takenfrom the llmiter output, such an arrangement in itself will provide automatic cuto of the noise suppression eiect as soon as a signal voltage of lust suillcient magnitude to saturate the limiter is received, by reason of the fact that at this point amplitude modulation in the limiter output is prevented and the squelch'voltage auto# matically collapses. It will be understood of course that noise voltagesmay also, if desired, be detected at the limiter input. In another modication this arrangement is used. Where such a system 4is usednolse voltages are still detectable even though .the limiter is saturated,

imiter.

appended claims.

so that special means must be provided to disable the squelch circuit when a received signal voltage attains the desired magnitude. -For the purpose of providing selectable cutoif of the squelch voltage for any predetermined carrier signal strength, I combine in o'pposing relation with the detected noise bias voltage a biasvoltage of controllable and substantially constant value to obtain a net or differential bias voltage of selectable value whereby by dim inution of the noise bias voltage with increasing carrier voltage the net squelch voltage may be brought to zero at any predetermined selectable signal level. A sensitive trigger action of the squelch circuit is insured by providing means for insuring immediate collapse of the differential bias voltage as soon as it tends to reverse due to predominance of the constant controllable bias. When such a differential voltage is used the noise voltages may be taken either at the l limiter input or output.

advantages thereof may best be understood by invention.

in frequencyor phase by desired signals, or irregular voltages such as a're characteristic of the types of noise.

By the term frequency modulation I mean to include 'any form of modulation produced by varying the frequency or phase of the carrier wave as distinguished from modulation produced by varying the amplitude of such a wave.

.In thediagrammatic representation of a frequency modulation receiver shown in Fig. l, many of the elements may be conventional and their details of construction arenot material to my Therefore to simplify Athe drawing and the description oi such elements, they have been shown merely in block form. The receiver illustrated provides a signal channel which is for the most part of the conventionalized superreferring to the following detailed description taken in conjunction with the accompanying drawing in which Fig. 1 is a schematic representation, partly in the form of a conventionalized oneline diagram, of a frequency modulation receiver embodying Amy invention: and Fig. 2 schemati-` cally represents a modied form of my invention,

some of the conventional portions of the frequency modulation receiver being omitted for simplicity. Fig. 1 has Ybeen utilized to illustrate two modifications of my invention by the interposition of 'a selector switch for choosing various circuit connections. 'In order to facilitate comparison o f the two figures and to simplify the accompanying description, like reference numerals have been placed upon corresponding elements of the tlg` urea. l

Before proceeding further with the description of the illustrated embodiments of my invention, I wish to explain the meaning of'certain terms al used hereinafter in the speciilcation and the I intend such terms as "noise or "noise Ivoltagee, unless otherwise qualified, to include al1 undesired voltages which may accompany the desired'sigals or be-modulated upon them, or which may exist in the absence of the desired signals. For example, these voltages may be transmission channel; they may be caused by thermal agitation' eifects, shot effects, and related phenomena which occur in the receiver itself and give' rise to disturbances of the socalled continuous'ior "hash type; or they may be caused by natural atmospheric static, 'ignition interference 'and the like which occur outside the receiver and give rise to disturbances ofthe socalled impulse type. Buch voltages normally vary ulti-utensili! or amplitude at irregular and rapid n By the lterm amplitude modulation" and the like I intend to include all amplitude variations in the envelope of high -frequency voltages appearing at specined points of tbe Jtransmission channel, irrespective of the precise character of these voltages. V'I'hus,for the purpose of my invention these voltages Imay constitute periodic signal voltages either unmodulated or modulated heterodyne type. Signals received at an antenna III are amplied in the usual radio frequency am pliilers il and combined in a mixer i2 with 1ocally generated waves supplied from an oscillator I3. 'I'he carrier waves of the resulting intermediate frequency are then amplified in intermediate frequency ampliers Ill. In frequency modulation receivers of the usual design it is often the practice to pass the intermediate fre# quency waves through two amplitude limiters "n cascade in order to securebetter limiting action. Thus in the illustrated embodiment of the inventfon some of the amplitude modulation is re- .moved in a met limiter Il, and practically all of scribed in Patent No. 2,121,103, Seeley. granted June 21', 1938.

The demodulated audio signals are reproduced by supplying them first to an audio frequencyv ampliner Il, further amplifying them in-an audio power amplifier il, and then supplying them to a suitable signal translating device such as a loudspeaker 20.

The second limiter I0 comprises a pentode amplifier tube Il having an input circuit including 'a coupling capacitor Il and a grid resistor I! connected between its control grid 33 and its cathode M, and an outputcircuit including a tuned circuit Il connected to its anode 38. Anode operating potentials are supplied in conventional 50 manner from a suitable source of power, lsuch as a battery 31, through the usual decoupling resistor 88. The screen grid l! of the pentode tube Il is similarly supplied with operating potentials from the common power supply source l1 through a decoupling resistor and is coupled to the cathode M by means of the usual screen grid by-pass condenser Il. The output circuit of the limiter It is also provided with a lay-pass condenser 42 of conventional form.

The operation of the limiter Il will be readily understood by those skilled in the art without further detailed explanation. The limiter is selfbiased by means of the capacitor 3i and the grid resistor 82. It is so adjusted as to draw anode vcurrent only between the limits .at which positive grid current flows and at which anode current cutoi! takes place.

As previously explained, the diagram of Fig. 1

' lis a combined illustration of two modifications of y 3 Nevertheless, under such conditions it has been found that these voltages are not appreciably circuit according to one modification of the invention, and when in the other position, connects the noise suppression circuit in accordance limited in amplitude in the limiter I6, and that substantial noise voltages are developed at the limiter output including modulation freqencies within the audio band.

As soon as a carrier signal is received which is sufiicient to maintain the limiterV I 6 continuously saturated, the amplitude modulated audio noise voltages appearing at the point 50 almost completely disappear so that the bias' voltage with a second modification. 'I'he form of the in-` vention resulting from positioning the selector switch 45 in the manner illustrated in Fig. 1 will first be described.

In the circuit of Fig. 1 the time constant of the grid circuit 3|, 32 is such that the pentode 30 functions essentially as a grid power detector for amplitude modulation components within the audio frequency band. Therefore it produces audio voltages across an audio frequency load in the anode circuit. Such a load is provided by the decoupling network comprising the resistor 38 and the capacitor 42. Accordingly, noise voltages at audio frequency appear at the'point 50. These voltages are supplied through a low pass filter comprising a resistor 56 and a. capacitor 51 and through a blocking condenser 55 to a diode rectifier 58. The low pass filter 56, 51 has for its purpose theV removal of intermediate frequency, while the condenser 55 removes any direct current in the limiter output. Any detected audio frequency voltages which are thus applied to the diode detector 58 are rectified and appear across a load resistor 59 as unidirectional voltages having a magnitude proportional to the degree of amplitude modulation present in the 'noise voltages. l

The unidirectional voltages appearing across the resistor 59 are filtered through a low pass filter comprising a resistor 60 and a capacitor 6| and impressed between a control grid 62 and the cathode 63 of a first audio amplifier tube 6d through a resistor 65. As indicated in the drawing, the voltages appearing across the load resistcr 59 are of such polarity as to bias the grid.

62 negatively with respect to the cathode 63 and, if the amplitude modulation present in the noise voltages at the output of the limiter I6 is suincient, `the bias voltages impressed upon the grid .62 are sufficient to reduce the amplification of the audio amplifier 64 to a very low level and preferably to bias it completely to cutoff, thereby reducing the transfer efliciency of or preventing transmission of signals to the sound reproducer comprising the power amplifier I9 and theloudspeaker 20.

I also provide a diode rectifier l connected across the terminals of the load resistor 59. Accordingly, when the noise voltages at 'the output of the limiter .I6 are sufficient to bias the audio ampliiler'grid 62 negative -with respect to its associated cathode 63, the plate il of the diode 'l0 is also biased negatively with respect tothe cathode 'l2 of this tube and the diode i0 does not conduct and has no eiect upon the operation. The function of the diode lil will be described hereinafter.

As indicated earlier in this specification, it is preferable to provide such a high gain through the radio frequency and intermediate frequency amplifiers that the noise voltages impressed upon the limiter grid 33 develop self-bias potentials approaching values required to saturate the pentode tube 3B even under rio-signal conditions.

across the resistor 59 completely collapses. How ever,it has been found that the audio bias .voltage developed across the resistor 59 under nosignal conditions is considerably more than sumcient to bias the audio amplifier tube 64 to cuto'. Consequently with the circuits thus far described it is necessary to-receive carrier voltages of substantially full strength sufiicient continuously to saturate the limiter I6 in order to reduce the negative bias of the grid 62 sufliciently to unblock the amplifier tube 64.` 'I'his is due to the fact that considerable noise voltage amplitude modulation may still exist in the output of the limiter I6 even though at relatively weak signal voltage is being received. l

Under certain circumstances such as in police i emergency receivers, it is desirable to provide means for disabling the squelch circuits upon the reception of even a weak signal voltage, so that though a certain degree of background noise is present. To provide for such a contingency, my

invention includes means for selectablyA controlling the amount of.cutoff bias supplied to the grid 62 of theamplier tube 64. By suitably adjusting the net gridbias under no-signal conditions, I provide Afor complete collapse of this bias upon any predetermined degree of diminution of the noise voltage appearing across the resistor 59 occasioned by the reception of a weak signal voltage.at.the output of the limiter I6. `'Io this end au substantially constant positive or opposing bias of selectable value is applied by means of an adjustable potentiometer l5 between the grid 62 and the cathode 63 of the-amplifier tube 64 from the battery-31 through a resistor 16. The purpose of the resistor` 16 is to-reduce the positive bias to a suitable value, and the resistor is preferably so selected that the maximum negative noise bias under no-signal conditions exceeds the normal positive bias by an amount just suflicient to bas'the tube 64 to cutoff. Thus, by Way of example, if ten volts of negative bias are applied to the grid 52 by the rectified noise voltages under rio-signal conditions, a positive bias of seven or eight volts may be applied from the potentiometer 15, so that the net bias of two or three volts negative is just suicient to maintain the amplifier tube 64 inoperative. Under such con@ ditions if a very weal signal voltage is received, thereby reducing the magnitude-of the rectified noise voltage only slightly, the net negative bias applied to the grid 62 Will be reduced suiilcientlyto render the amplifier tube G4 operative. Similarly, itwill be evident that by decreasing the amount of positive bias from the resistor 15, the l differential or net negative biasapplied to the necessary strength'of signal voltage for rendering the amplifler tube 64 conductive may be selected.

The function of the diode rectifier 10 will now become apparent. It will be clear that, if the positive grid ybias from the potentiometer "l5 is so of the diode rectifier 10 precludes such sufficient continuously to saturate the limiter I6, l

the negative noise voltage appearing across the resistor 59 would be reduced to zero and the full positive bias from the potentiometer 15 would be left upon the amplifier grid 62. II'he presence a. possibility in that, as soon as the net bias between the audio amplifier grid 62 and the audio amplifier o cathode Il goes positive, the plate 1l of the diode will also assume a positive voltage with respect to the associatedl cathode 12, and the diode will conduct. Since the internal resistance of the diode 1li when conducting is very small compared with the magnitudes of the resistors Il and 18, any positive bias which might otherwise be supplied to the grid I2 from the potenq tiometer substantially vanishes.

Accordingly. it will now be evident that I have provided means whereby rectified noise voltages at the limiter stage may be differentially combined with an opposing bias voltage, thereby to adjust the maximum cutoff bias of a subsequent stage in the signal channel.' such as an audio ampliner, to any desired value and thus select the carrier signal strength at which it is desired to have the cutoff bias of the amplifier stage disabled. My invention also provides means for insuring sensitive operation by maintaining the net amplifier grid voltage substantially at zeroeven thoughthe rectied noise voltage itself goes torero, thereby preventing reversal of the net or differential grid voltaee.

The modified form of myinvention represented at Fig. l with the selector switch transferred from the .position shown in the vdrawing to the contact Il differs from that described above only in certain'particulars. Accordingto this modification of the invention the source of positive potential from which a positive bias is applied to the grid l! of the ampliner tube through the potentiometer 1B is shown as a point in the limiter output circuit rather than the positive terminal of the battery 31.- Specifically, the potentiometer Il is connected to the plate return of the limiter Il through a conductor Il and a ,low pass nlter comprising a resistor i! and a condenser I3. This modification of the invention has the additional advantage that the positive potential of the plate 30 ofthe limiter Il. and therefore the positive potential applied to the control grid 'l2 of the audio amplifier Il, `increasesgsimultaneeusly with a decrease in the negative noise bias across the load resistor Il of` the squelch circuit. Thus, the net squelch bias on theA ampli- I der tube 04 approaches pero more rapidly as soon as a weak carrier voltage is received, thereby further reducing the minimum carrier input necessary .to ldisable the squelch circuit. l

. At Fig. 2, I' have shown a modified form of my invention embodying certain refinements which have for their purpose an assurance thatfa) the squelch circuit shall not cease to function in the presence of either imsmlseor continuous, with a lack of signal or carrier voitthe various elements of the pentode 30a being have! .put. of the squelch amplifier.

astenia U age; (b) the squelch circuit shall not function in the presence of a. predetermined minimum signal voltage; (c) the squelch circuit shall not be set in operation by the presence of a reasonable amount of amplitude modulation in the signal voltage. Briefly, these purposes are accomplished by receiving the noise indication from the grid of the first limiter and applying noise voltages thus received to the squelch circuit through a special squelch amplifier in combination with a high pass filter for removing low frequency components from the amplifier output.

Referring now more specifically to the diagram of Fig. 2, it will be noted that the circuit there shown is in many respects similar to that illustrated in Fig. 1, and to simplify the description and facilitate a ready understanding of the invention, corresponding elements of the system been assigned like reference numerals. Voltages appearing atthe control grid 33a of the first limiter I5 are utilized to set the squelch circuit in operation. The first limiter comprises a pentode tube 30a and a tuned output circuit 35a,

assigned reference numerals similar to those designating the corresponding elements of the pentode 30 of Fig. l. -Voltages appearing at the grid 33a may be either frequency modulated Acarrierl signal voltages, amplitude modulated Anoise voltages, or both. As shown, the limiter grid voltage at' 33a is applied to a control lgrid 90 of a squelch amplifier 'triode 9| through a voltage divider comprising a plurality of resistors 92, Il, and 94 connected in series circuit relation betweenthe control grid 33a and ground. The rere'sistors 93 and 94 are shunted by a capacitor DI, which, in combination with the resistor l2 and certain stray capacities present, serves as a low pass filter for removing voltagesv of intermediate frequency. Thus, only audio frequency voltages are applied to the grid of the squelch amplifier 9i. An audio frequency bypass condenser lla shunts the resistor 93. A

If now it is assumed that no carrier-signal is present and that only amplitude modulated noise voltages are present at the limiter grid 33a, the audio frequency components of these voltages will be amplified by the squelch amplifier Il. The anode-cathode circuit of the amplifier 9i is supplied from a suitable source of direct current supply such as the battery31. This output cir# cuit also includes an iron core choke coil l1 which with thevcondenser 55 acts as a high passnlter to remve from the output of the amplifier tube li any low frequency components due to amplitude modulation of the audio frequency out- In this manner amplined and detected audio frequency'voltages yare applied to the diode detector 58 and are rectified to develop across the load resistor Il a negative bias potential of a magnitude proportional to the degree of amplitude modulation present in the noise voltages at the limiter grid um The negative noise potential developed across the load resistor il is filtered in the low pass filter Il, Il and supplied through the resistor I! to the `control grid B2 of the first audio amplifier tube I4 in the same manner as has been hereto fore. explained with reference to Fig. 1. However.

due to the action of the squelch ampliner tube y Il, the magnitude of the negative bias voltage thus applied to the grid 82 is considerably greater'4 than that developed with a circuit arrangement similar to that of 1.

Accordingly,J for the 'purpose of reducing the.'

squelch amplifier 9|.

cutoff potential applied to the grid 82 fof the plifler tube 64 and also for the purpose of provlding an adjustable and sensitive or triggerllke operation as heretoforeexplained, the negative bias developed across the load resisto;` 59 is opposed by a positive bias of slightly less magnitude applied to theamplifier grid 62 from the supply source 31 through a variable squelcl'i potentiometer 15, a. As heretofore explained, the positive bias is reduced to a suitable normal value by a resistor 16. The reverse potential short-circuiting diode 10 serves the same purpose'as has been heretofore explained with reference to Fig. 1. y

If now a signal voltage of substantially constant magnitude, but modulated with respect t`o lter including the choke 91 precludes closing of the .squelch due tov modulation ot otherwise in-y cycles. In this particular receiver the -tube a frequency in accordance with the intelligence being transmitted, is appliedy to the grid 33a oi' the limiter l5 along with certain amplitude modulated noise voltages, the increased D. C. volt age applies an increased negative bias to the grid 90 of the squelch amplifier 9|, thereby tending to cut on the amplifier. While the developed negative bias may not be suicient to cut off the squelch amplifier 9|', it will be sufficient to decrease the amplifier gain suiiciently so that the developed output voltage at audio frequency is reduced, thus reducing the negative bias across the load resistor 59 and the net negative bias on the grid 62 of the audio amplifier 64. It will of course be understood that, as in the modification of Fig. 1,k the: net negative bias on the grid 62 under .no-signal conditions may be controlled by means of the potentiometer 15, 15a, thereby selectably to' determine the minimum signal strength at which the squelch control opens or becomes ineffective. Also, the manner in which v the diode l0 acts to preclude the imposition upon the control grid 62 of a net positive bias, thereby to insure rapid and permanent collapse of the net squelch bias at a selected cutoff point so long as the noise voltage remains reduced, has also been explained with reference to Fig. 1.

The circuit of Fig. 2 isl so arranged that an increased negative bias applied to the .control grid 90 of the squelch amplifier 9| without thepresence of a signal voltage, as increased direct current bias due to increased extraneous noise, will not tend to unblock the first amplifier tube 64. For this purpose the voltage dividing resistors 93 and 98 are so selected that an increase in the negative bias applied to the squelch amplifier 9| by the application of increased noise voltages to the limiter is balanced by the increase in the audio noise voltage supplied through the bypass condenser- 93d to the control grid 90 of the That is to say, the in'- creased negative or cutoff bias appliedo to the grid 90 is eounteracted by the greater audio frey quency voltage applied to' the grid, thereby to maintain the developed squelcn bias across the load resistor 59 substantially constant in magnitude. It will be understood that l'increase in the negative bias on the grid 90 due to the presence of a signal voltage of substantially constant magnitude will provide the tendency .to cut oil the squelch amplifier 9|, as previously described. but due to the fact that the audio frequency volt- Y age is not thereby increased, it will not provide the counterbalancing tendency present with increasing noise voltages. Thus there is neither a tendency to set the squelch circuit in operation in the presence of a frequency modulated signal voltage, nor a tendency to open the SquelchI with increased noise. Furthermore, the high pas.

was' a type 7C? pentode, the diode 59 and the squelch ampliner 9| were embodied in a single duodiode nigh mu triode type 706, and the audio frequency amplifier 64 and the diode l0 were embodied in a second 706 type tube. Other circuit constants were as follows:

While I have shown certain preferred embodiments of my invention by way of illustration, it will of course be understood that I do not wish to be limited thereto since'various modifications will occur to those skilled in the art. I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention. p

What I claim as new and desire to secure by letters Patent of the United- States, is:

1. In a radio receiving apparatus, a signal channel adapted to transmit a signal wave modulated in frequency within a band of frequencies in accordance with an impressed signal, said channel being also eiective to transmit noise voltages appearing as audio frequency amplitude modulation of said signal wave, signal reproducing means coupled to said channel, means responsive to said amplitude modulation of said signal wave for at least partially disabling said'sig.

predetermined minimum amplitude.

2. In a radio receiving apparatus, a signal amplifier adapted to amplify a carrier wave modu iated in frequency withinV a band of frequencies in'accordance with an impressed signal, said nimmer being also effective w supply in its outf lput substantial noise voltages modulated in amplitude at audio frequency, signal reproducing means coupled to said amplier, means for developing a lunidirectional potential proportional v in value to the degr'eeof amplitude modulation of waves supplied by'said amplifier. means for ,reducing .said `potential in accordance with in- -lated in frequency stantially to zero when said waves attain a prel determined level, means for selecting said predetermined level comprising a second unidirectional potential of selectable value opposing said ilrst potential thereby to provide a selectable and variable differential potential, and means controlled by said dierential potential for atleast partially disabling said signal reproducing means thereby to preclude the reproduction of noise voltages so long as said carrier wave is below said predetermined level.

3. En a radio receiving apparatus, o. signal amplifier adapted to amplify a carrier wave modull in frequency within a band o irequem cies accordance with an impressed signal, said amplifier being also eective to supply in its output substantial noise voltages modulated in amplitude at audio frequency, an amplitude limiter coupled to said amplifier, a signal reprodncer coupled to said limiter, moans associated with said limiter for developing a unidirectional potential proportional in value to the degree of amplitude modulation present in waves below a predetermined level supplied to said limiter, ad-

' instable means for opposing said potential with a second unidirectional potential of selectable value thereby to provide a differential potential, and Imeans controlled by said differential potential for at least partially disabling said signal reproducer, said adjustable means being operable selectably to render said disabling means ineffective in Ithe presence of carrier waves at any level below said predetermined level.

4.1m a radio receiving apparatus, a signal amplifier adapted to amplify a carrier wave modulated in frequency within a band of frequencles in accordance with an impressed signal, said amplier being also eflective to supply in its output substantial noise voltages modulated in amplitude at audio frequency, an amplitude limiter coupled-to said amplifier, signal reproducing means including an electron discharge de-` vice having a control electrode coupled to the trode coupled to the output of'said limiterl amplitude modulation detection means for developing a negative bias potential upon said control electrode proportional tol amplitude modulation of waves supplied to said limiter below said limiting level, and means for supplying to said control electrode a positive bias potential of selectable value less than said negative potential so long as the amplitude of said carrier wave remains below a desired value thereby to suply to said control electrode a net negative alas potential sulcient substantially to disable said reproducing means until said carrier wave exceeds said desired value. I

6. ln. a radio receiving apparatus, a signal ampli'ler adapted to amplify a carrier wave modulated in frequency within a band of frequencies in accordance with an impressed signal, said amplifier being also eective to supply in its output substantial noise voltages modulated in amplitude at audio frequency, an amplitude limiter coupled to said amplifier, signal reproducing means including 'an electron discharge device having a control electrode coupled to said limiter, means associated with said limiter for developing a, unidirectioal potential proportional in value to the degree of amplitude modulation present in waves below a predetermined level supplied to said limiter, adjustable means for opposing output of said limiter, means responsive to amplitude modulation of waves below a predetermined level supplied to said limiter to apply to said control electrode a bias potential proportional to said amplitudeinodulation, anddiustable means for applying to said control electrode an opposing bias potential of selectable value thereby to provide a diilerential bias potential of such polarity as to tend to disable saidelectron discharge device, said adjustable means being arranged to maintain said differential potential sunlcient substantially to disable nid electron discharge device in the absence of a carrier wave and selectably to reduce said diflerential potential to zero in the presence of carrier waves below said predetermined level;

5. In a radio receiving apparatus, a plifler adapted to signal am# within a band of frequencies in accordance with an impressed signal, said ampliiler being also effective to supply in its output 'substantial undesired noise voltages modulated in amplitude at audio frequency, an amplitude limiter coupled toibaid ampliner, said limiter being effective to remove: amplitude moduf lation at carrier frequencies from thewaveupplied to said limiter so long asa carrier wave exc'eeding a predetermined limiting level in supplied thereto,'a signal-reproducer including an l electron 'discbargedevice having a control elecamplify a carrier wave modu-1 said potential with a second unidirectional potential of selectable value thereby to 'provide a differential potential, means for applying said differential potential to said control electrode to disable said electron discharge device, said adjustable means being operable selectably to render said disabling means ineffective in the presence of carrier waves below said predetermined level, and means for preventing reversal of said differential potential upon diminution of said first-mentioned unidirectional potential.

7. In a radio receiving apparatus, a signalempliiler adapted to amplify a carrier wave modulated in frequency within a band of frequencies in accordance with an impressed signal, said ampliiler being also eilective to supply in its output substantial noise voltages modulated in amplitude at audio frequency, an amplitude limiter coupled to said amplifier. signal reproducing means including an electron discharge device having 'a control electrode coupled to said limiter, means associated with said limiter for developing a negative bias potential proportional in value to the degree of amplitude modulation present in waves below a predetermined level supplied to 'carricr waves below said predetermined level,

means for applying said differential bias potential to said control electrode to reduce the tramfer emciency of said electron discharge device, and means tor' preventing reversal of said dinerential potential comprising a second electron discharge device connected normally'to be nonconductive and tobecome conductive upon any tendency of said diilerential potential to reverse.

Vil. In a radio receiving apparatus, a signalantpliner adapted to amplify a carrier wavemodulated in frequencyl within a band of frequencies in accordance with an impreascdlimll. said ampliiier being also elective to supply lin ih out- ,a

asvaaie' put substantial noise voltages modulated in 4am'- plitude at audio frequency, an amplitude limiter coupled to said amplifier, signal reproducing means including an electron discharge device having a control electrode coupled to the output of said limiter, means responsive to amplitude modulation ofNaves below a predetermined level supplied to said' limiter to apply to said control electrode a bias potential proportional to said amplitude modulation, adjustable means for applying to said control electrode an opposing bias potential of selectable rsubstantially. constant value thereby to provide a differential biaspotential of such polarity as to tend to disable said electron discharge device, said adjustable means belngarranged to maintain said differential potential suiiicient substantially to disable said electron discharge device in the absence of a carrier wave and selectably to reduce said differential potential to zero in the presence of carrier waves below said predetermined level, and means associated with said electron discharge device for preventing a reversal of said differential bias potential.

9. In a radio receiving apparatus, a signal ampliler adapted to amplify a carrier Wave modulated in frequency within 'a band of frequencies in accordance with an impressed signal, said amelectron discharge device, the magnitude of said differential potential in the absence of a carrier wave being selectable by said adjustable means thereby to provide for reduction of said differential -potential to zero in the presence of a carrier wave' of desired amplitude.

11. In a radio receiving apparatus, a signal amplifier adapted' to amplify a carrier wave modulated in frequency within a band of frequencies in accordance with an impressed signal, said amplier being also effective to supply in its output substantial undesired noise voltages modulated in amplitude at audio frequency, an amplitude limiter coupled to said ampliner, said limiter including an electron discharge device having an output circuit, a signal reproducer including a second electron discharge device having a control electrode coupled to said output circuit,

vamplitude Amodulation detection means also plifler Vbeing also' effective to supplyfin its output substantial undesired noise voltages modulated in amplitude at audio frequency, an amplitude limiter coupled to said amplifier, said limiter comprising an electron discharge device having an output circuit and being eiective to remove amplitude modulation at carrier frequencies from the wave supplied to said limiter so long as a carrier wave exceeding a predetermined limiting level is supplied thereto, a signal reproducer including a second electron discharge device having a control electrode coupled to the output of said limiter, amplitude modulation detection `means for developing a negative bias potential upon said control electrode proportional to. amplitude modulation of waves supplied to said limiter below said limiting level, means associated with said output circuit of said first electron discharge device for supplying to said control electrode a positive bias potential of selectable value less than said negative potential as long as the amplitude of said carrier wave remains below a desired value, and means responsive to a predominance of said positive bias potential to maintain the net potential upon said control electrode substantially at zero.

10. In a -radio receiving apparatus, a ,signal amplifier adapted to amplify a carrier wave modulated in frequency within a. band of frequencies in accordance with an impressed signal, saldamplier being' also effective to supply in its output substantial noise voltages modulated in amplitude at audio-frequencman amplitude limiter coupled to said amplifier, said limiter comprising an electron discharge 'device having an output circuit, signal reproducing means including,- a second electron discharge device having 4a con-- trol electrode coupled to said output circuit', means responsive to amplitude modulation of waves appearing at the output of said limiter for applying to said controlelectrode abias potential of proportional Value, adjustable means for applying to said control electrode an opposing biasof selectable substantially constant value. A

thereby to provide'a differential bias potential of such polarity as to tend to disable said vsecond coupled to the output of said limiter for devel-- oping a negative bias potential upon said control electrode proportional to amplitude modulation of waves appearing in said output circuit, means for supplying to said control electrode a positive bias potential of selectable substantially constant value less than said negative potential as long as the amplitude of said carrier wave remains be- Vin amplitude at audio frequency, an amplitude limiter having an output circuit coupled to said amplier, signal reproducing means including an y electron discharge device having a control electrode coupledto said output circuit, amplitude detection means also coupled to said voutput circuit and arranged to apply to said control electrode a negative bias potential proportional to the degree of amplitude modulation of waves appearing in said output circuit, adjustable means for supplying to said control electrode from a point in said output circuit an opposing positive bias potential of selectable value thereby low a desired value, and means ior preventing to provide a dierential bias potential of such polarity as to tend to disable said electron discharge device, the magnitude of said differential potential in the absence of a carrier wave being normally sufiicient completely to disable said Yelectron discharge device and being selectable by said adjustable means to provide for reduction thereof-to zero in the` presence of a carrier wave of any desired value." y

13. In a radio receiving apparatus, a signal amplifier adapted 'to amplify a carrier wave modulated in frequency' within a band of frequencies in accordancevwith an impressed signal, said amplifier being also eective to supply -in its output substantial noise voltages modulated in amplitude at audio frequency, an amplitude limiter coupled to said ampliner including an electron discharge device having an anode circuit, signal reproducing vmeans including a second electron discharge device having a control electrode coupled to said anode circuit, amplitude detection means also coupled to said anode circuit for applying to said control electrode a negative bias potential Prwortional to' the degree of amplitude modulation of'waves appearing in said anode circuit, and adjustable 'means coupled to the anode of said ilrst electron discharge device for applying to said control electrode ana opposing bias potential of selectable value thereby to provide a differential bias upon said control electrode tending to disable said electron discharge device, the magnitude of said differential potential in the absence of a carrier Wave being normally suicient completely to disable said electron discharge device and being selectable by said adjustable means to provide for reduction thereof to zero in the presence o a carrier wave of any desired value.

14. In a radio receiving apparatus, a signal amplier adapted to amplify a carrier Wave modulated in frequency Within a band of frequencies in accordance with an impressed signal, said amplifier being'also effective to supply in its output substantial noise voltages modulated in amplitude at audio frequency, signal repron ducing means coupled to the output of said amplier, means including an electron discharge device for amplifying and detecting amplitude modulated voltages appearing in the output of said amplifier, said means including means to reduce the gain of said electron discharge device only in the presence of a carrier wave, means for developing a potential proportional to said detected voltages, means for opposing said potential to provide a differential potential, and means responsive to said differential potential for at least partially disabling said signal reproducing means. Y

15. In a radio receiving apparatus, a signal amplifier adapted to amplify a carrier wave modulated in frequency.within a band of frequencies in accordance with an impressed signal,

said amplifier being also effective to supply in its output substantial noise voltages modulated in amplitude at audio frequency, signal reproducing means coupled to the output f said amplifier, means `including an electron discharge device for amplifying and detecting audio frequency voltages appearing in the output of said amplifier.

said means including means to'redce the gain of said electron discharge device in the presence of a carrier wave, said gain remaining substantially constant upon increase of said noise voltages, means fordeveloping a potential 'proportional to said'detected audio frequency voltages,

selector means for opposing said potential to provide a differential potential, means responsive to said differential potential for at least partially disabling said signal reproducing means, and means associated with said slgnalreproducing means for preventing a. reversal ofsaid differential potential.

" i -16. In a radio receiving apparatus, a signal amplifier adapted to amplify carrier waves modulated in frequency within a band of frequencies in accordance with an impressed signal, said amr pllner being also effective to supply in its output substantial noise voltages modulated in amplitude.- at audio frequency,v signal reproducing means coupled to said amplifier, means includins asf/0,216

noise voltages, means for developing a potential proportional to said detected audio frequency voltages, means rendering said last-named means nonrespo-nsive to amplitude modulation of said detected audio frequency voltages, selector means for opposing said potential to provide a differential potential, means responsive to said differential potential for at least partially disabling said signal reproducing means, and means associated with said signal reproducing means for preventing a reversal of said dierential potential.

17. In a radio receiving apparatus, a signal amplifier adapted to amplify a carrier wave modulated in frequency within a band'of frequencies in accordance with an impressed signal, said amplifier being also effective to supply in its output substantial noise voltages modulated in arnplitude at audio frequency, signal reproducing means coupled to the output of said amplifier, means including an electron discharge device having a control electrode for amplifying and detecting 'audio frequency waves appearing in the output of said amplifier, the output of said amplifier being coupled to said cmtrol electrode through a circuit so proportioned that the output of said electron discharge deviceremains substantially constant with variations in intensity of said noise voltages, said circuit being arranged to reduce the gain of said electron discharge device in the presence of a carrier wave of predetermined intensity, means for developing a potial proportional to said detected audio frequency Wave, selector means for opposing said potential to provide a differential potential, and means responsive to said differential potential for at least partially disabling said signal reproducing means. y

18. In a. radio receiving apparatus, a signal amplifier adapted to amplify a carrier wave modulated in frequency within a band of frequencies in accordance with an impressed signal, said amplifier being also effective to supply in its output substantial noise voltages 4modulated in amplitude at audio frequency, signal reproducing means coupled to said amplifier and including a first electron discharge device having a control electrode, means including a second electron discharge device having a. control electrode for detecting and amplifying audio frequency waves appearing in the putput of said amplifier, thev said detected audio frequency wave, selector means for applying to the control electrode of said first electroresponsive device a positive bias potential less than said negative bias potential u so long as the intensity -of said carrier wave an electron discharge device for' amplifying and detecting audio frequency voltages appearing in the output of said amplinenssid means including means for reducing the gain offsaid electron.

discharge device in the presenceI of s carrier wave of predetermined` amplitude,saidl vgain remainins substantially unaffected byY .of said gif" remains below a predetermined level, and unidirectional. conducting means coupled to said first electron ldischarge device for preventing c reversal of the net bias potential applied to said.

first electron discharge device. l

.lossen A. woacks'm, Jn.l v 

